The present invention relates to efficient and easy to operate distillation schemes to separate multicomponent mixtures containing three or more components into product streams each enriched in one of the components. Generally, the objective of a process engineer designing a distillation scheme is to make it more efficient by reducing the heat requirement of the distillation columns within the distillation scheme. The distillation schemes known in the literature that require lower heat duty are quite complex and difficult to operate. As a result, many of these schemes lack operating flexibility and are rarely used in industry. Therefore, there is a need for distillation schemes that are easy to operate while having low heat requirements. The present invention is an answer to this long desired need for improving the operating flexibility of multicomponent distillation while maintaining lower heat duties.
Consider the separation of a ternary mixture having components A, B and C (mixture ABC) into three product streams each enriched in one of the components. A is the most volatile component and C is the least volatile component. To separate a ternary mixture ABC into almost pure components it is required that a distillation scheme use two distillation columns. Such distillation schemes are well known in the art. There are such five well known schemes: direct sequence, indirect sequence, side rectifier, side stripper and fully thermally coupled columns (for examples see page 711 of the book entitled Separation Processes by C. J. King, McGraw-Hill, 1981). It is further known that of all the known ternary distillation schemes, the fully thermally coupled column system requires the least amount of heat duty (see "Minimum Energy Requirements of Thermally Coupled Distillation Systems", Z. Fidkowski and L. Krolikowski, AlChE Journal, pages 643-653, volume 33, 1987). In spite of this attractive performance, the fully thermally coupled column system has not been widely used in commercial applications. A description of this scheme, along with associated operating problems will now be given.
FIG. 1 shows a fully thermally coupled column (FC) scheme. A feed mixture containing components A, B and C (stream 10) is fed to first distillation column 100. In this distillation column, the feed stream is separated into two streams that are primarily binary mixtures. The liquid from the bottom of this distillation column (stream 22) is primarily a binary mixture composed of components B and C. Similarly, the vapor from the top of this distillation column (stream 32) is primarily a binary mixture composed of components A and B. Both these primarily binary mixture streams 22 and 32 are fed to different locations of a second distillation column 200. A portion of the liquid from the bottom of second distillation column 200 is recovered as C-enriched product stream 90, and another portion of this liquid is boiled in reboiler 222 and returned as vapor stream 92 to provide boilup for second distillation column 200. The vapor from the top of second distillation column 200 is condensed in condenser 212, a portion is recovered as A-enriched product stream 70 while the other portion is returned to provide the needed liquid reflux for this distillation column. A B-enriched product stream 80 is produced from an intermediate location of second distillation column 200. This withdrawal location is somewhere in between the two primarily binary feed streams 22 and 32. The first distillation column 100 neither uses a reboiler nor a condenser. The boilup at the bottom of this column is provided by feeding a vapor stream 27 from the second distillation column 200. It is important to note that the withdrawal location of vapor stream 27 is from the same location of the second distillation column 200 as the feed location of the primarily binary liquid stream 22. This leads to a two-way communication between the two distillation columns. In a two-way communication mode, when a vapor stream is sent from one column to another column, then a return liquid stream is implemented between the same locations of the two columns. Similarly, the liquid reflux stream 37 to the top of the first distillation column 100 forms another two-way communication between the two distillation columns and is withdrawn from second distillation column 200. It is taught in the prior art that two two-way communications are needed to achieve the lowest heat demand for ternary distillation.
While the heat demand for the scheme in FIG. 1 with two two-way communications is lowest, it has rarely been used. The lack of use has often been attributed to fear of control problems (see "Thermal Coupling for Energy Efficiency", H. Rudd, Supplement to the Chemical Engineer, pages S14-S15, Aug. 27, 1992, and "The Design and Optimization of Fully Thermally Coupled Distillation Columns", C. Triantafyllou and R. Smith, Trans. IChemE, pages 118-132, Volume 70(A), 1992). One of the often cited concern is the flexibility to control the flows over a wide range both at the top and bottom ends of first distillation column 100. For the vapor AB in stream 32 to flow from first distillation column 100 to second distillation column 200 it is required that the pressure at the top of the first column be greater than the pressure at the feed point of stream 32 in second distillation column 200. At the same time, for the vapor BC in stream 27 to flow from second distillation column 200 to first distillation column 100 it is necessary that the pressure at the bottom of first distillation column 100 be lower than the pressure at the originating point of stream 27 in the second column. This leads to an unique restriction that the pressure at the bottom of the first distillation column 100 be lower than the pressure at a point in the bottom section of the second distillation column 200, and at the same time, the pressure at the top of the first distillation column must be higher than the pressure at a point in the top section of the second distillation column. This requires careful adjustment of pressure in both the columns and presents operating concerns for plants requiring wide range of variation in flow rate and other operating parameters. Clearly, there is a need for alternative column arrangements with higher operating flexibility while maintaining lower heat demand for distillation.
It is worth noting that both the liquid transfer streams 37 and 22 at the top and bottom of first distillation column 100 flow in a direction opposite to the vapor flow at each end. This requires that either a pump be used on each of the liquid streams or relative height of the two columns be adjusted to allow each of the liquid stream to be transferred through gravity.
Recently, Agrawal and Fidkowski introduced the scheme shown in FIG. 2 (see U.S. Ser. No. 09/057,211 filed on Apr. 8, 1998, now U.S. Pat. No. 5,970,472). In this figure, the bottom end of first distillation column 100 has a two-way communication with the bottom section of second distillation column 200 and the top end of first distillation column 100 has only one-way communication with the top section of second distillation column 200. Thus, liquid stream 22 from the bottom end of first distillation column 100 is sent to the bottom section of second distillation column 200. A vapor stream 27 is withdrawn from the second distillation column 200 and sent to the bottom of first distillation column 100. A portion of the vapor stream exiting from the top end of first distillation column 100 is sent as stream 32 to second distillation column 200. Unlike FIG. 1, there is no liquid return stream to the top of the first distillation column from the second distillation column. Instead, a portion of the vapor stream from the top of the first distillation column 100 (stream 34) is condensed in condenser 115 and returned as liquid reflux in line 36. Once again, a vapor stream is transferred from the first distillation column to the second distillation column and a second vapor stream is transferred in the reverse direction. This leads to the same operating challenges as described for the scheme in FIG. 1.
The same challenge exists when mixtures containing more than three components are distilled to produce product streams each enriched in one of the components. The reason being that the distillation schemes with low heat demand used to distill mixtures with more than three components are made up of the ternary subschemes shown in FIGS. 1 and/or 2. Therefore deficiencies of the ternary subschemes are also carried to the distillation of mixtures containing a greater number of components. Some known examples are four and five component distillation schemes can be found in a paper by Agrawal ("Synthesis of Distillation Column Configurations for a Multicomponent Separation", Ind. Eng. Chem. Res., volume 35, pages 1059-1071, 1996) and a paper by Sargent ("A Functional Approach to Process Synthesis and its Application to Distillation Systems", Computers Chem. Eng., volume 22, pages 31-45, 1998).
A sequential four-component separation scheme with at least four two-way communications is shown in FIG. 3. The feed mixture ABCD is distilled into four product streams. In this mixture, the relative volatility follows the alphabetical order, i.e., A is the most volatile, D is the least volatile and B is more volatile than C. The first column has two two-way communications with the second column which in turn has at least two (generally three) more two-way communications with the final column. Clearly, the challenges associated with the vapor transfers between the columns are now much greater as the pressure profiles in all the three column must be carefully controlled.
A four-component separation scheme with satellite column arrangement is shown in FIG. 4. The feed mixture ABCD is fed to the main column. There are two satellite columns each with two two-way communications with the main column. There is shown a possibility of having a liquid and vapor flow between the two satellite columns. The product stream enriched in the most volatile component A is produced from the top of the main column and the product stream enriched in the heaviest component D is produced from the bottom of this column. Product streams enriched in components of intermediate volatility are produced from each of the satellite columns. For each satellite column, in order to transfer vapor streams between columns, the pressure at the bottom must be lower than the pressure at a point in the bottom section of the main column and simultaneously, the pressures at the top of the satellite columns must be greater than the pressure in the top corresponding section of the main column. Furthermore, pressures of each of the satellite columns has to be adjusted to allow the flow of vapor and liquid streams enriched in component B and C in proper direction between the two satellite columns. All this presents a great deal of difficulty in the operation of such integrated schemes with multiple two-way communications between the distillation columns.